A liquid crystal panel included in a liquid crystal display device does not emit light, and thus a backlight device is required as a separate lighting device. The backlight device is arranged behind the liquid crystal panel (i.e., on a side opposite from a display surface side). It includes a chassis having an opening on a liquid crystal panel side, a plurality of fluorescent tubes (for example, cold cathode tubes) accommodated in the chassis as lamps, and an optical member (a diffuser plate and the like) provided at the opening of the chassis for efficiently directing light emitted from the cold cathode tubes to a liquid crystal panel.
In such a backlight device including cold cathode tubes emitting linear light, the optical member converts linear light into planer light to unify illumination light. However, if the linear light is not sufficiently converted into the planer light, striped lamp images are generated along the arrangement of the cold cathode tubes, and this deteriorates display quality of the liquid crystal display device.
To obtain uniform illumination light from the backlight device, it is desirable to increase the number of cold cathode tubes and reduce a distance between the adjacent cold cathode tubes or to increase a diffusion rate of a diffuser plate, for example. However, increase of the number of cold cathode tubes increases a cost of the backlight device and also increases power consumption. Increase of the diffusion rate of the diffuser plate fails to improve brightness and causes the problem that the number of cold cathode tubes is required to be increased. A backlight device disclosed in Patent Document 1 has been known as one that suppresses power consumption and ensures uniform brightness.
The backlight device described in Patent Document 1 includes a diffuser plate provided on a light output side of a plurality of cold cathode tubes. A dimming dot pattern having a light transmission rate (opening rate) from 62 to 71% and haze from 90 to 99% is printed on the light diffuser plate. A dot diameter of each dot is great directly above the cold cathode tubes and the dot diameter becomes smaller as is farther from the cold cathode tubes. With such a configuration, the light emitted from the cold cathode tubes is efficiently used and the backlight device irradiates light having a sufficient brightness value and uniform brightness without increasing power consumption of the light source.    [Patent Document 1] Japanese Unexamined Patent Publication No. 2005-117023